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01-17-2011, 10:37 AM   #1
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SMC-A 50mm f/1.7 with Internal Reflections in Back-lit Conditions?

Hi all,

I hope that I've found the right location for this question - I'm a long time lurker, first time poster. I'm relatively new to Pentax/DSLR's (shot with a film Nikon FE and a Minolta Maxxum when in school).

I recently purchased a used SMC-A 50mm f/1.7. Things looked pretty good - very clean, sharp images, etc. However, I've since found that high contrast back-lit scenes give the K-x body and 50mm lens a fit.

I've attached an example image. In the upper left there are blinds with bright back-lighting coming through. In the lower right there is a ghost of the light and blinds circularly symmetric to the original.

Things I've looked into:

1) Lens seems clean - nothing noticeable on either front or back elements, not much dust internally.
2) Included UV filter has no impact - ghosting/reflection is there whether the filter is on or off.
3) "Hand hood" doesn't change anything - perhaps an actual hood (rectangular or circular) would?

Does anyone have thoughts on this? Is this a normal experience for heavily back-lit images on this lens or with back-lit images in general?

Many thanks for any thoughts that could shed some light on this situation,
Josh


The specifics of the image:
Taken on Pentax K-x
No flash
SMC-A 50mm f/1.7 lens
1/125 second exposure
f/2.8 aperture
Automatic white balance
ISO 800
Firmware ver 1.01

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01-17-2011, 11:35 AM   #2
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I have the M version of this lens which I have used quite a bit, including in backlit conditions, and have never had such a problem with it. It really does look like ghosting from the UV filter. I know you say that the UV filter doesn't make a difference, but my advise is to just remove the filter and get a proper hood for the lens (the best one, in my opinion, is the Takumar 135mm f3.5/ 150mm f4 metal hood--nice and deep without vignetting, cheep, and you can put a 62mm cap on the front of it). Then see if you still have a problem.
01-17-2011, 12:41 PM   #3
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Looks like sensor reflection off the rear element. Not much you can do except to try a different shooting angle relative to the light source.
Any lens with a rear element close to the sensor can be problematic.
01-17-2011, 01:09 PM   #4
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I have this lens and have not seen this problem. I do not use a filter, but I do use a hood. I'd try similar shots w/o the filter and with a hood. I use an inexpensive rubber collapsable screw in filter, I think it cost me less than $5.

NaCl(let us know how you make out)H2O

01-17-2011, 01:41 PM   #5
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I would suspect that as sensor reflection too since DA lenses are specially coated on the rear element to prevent this.
I did some editing and you can see that the shadow outlined in red is the adult in blue albeit in reverse since it is a reflection. The glare of the blinds are reflected too although in white.

I super-imposed a copy of the guy in blue and pasted it on the reflected shadow so you can see a perfect fit.
The surrounding glare is the reflection of the blinds.

Hoods might help but not if the reflection or glare is head on since this would register on the rear element of the lens.
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01-17-2011, 11:25 PM   #6
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QuoteOriginally posted by GerryL Quote
I would suspect that as sensor reflection too since DA lenses are specially coated on the rear element to prevent this.
I did some editing and you can see that the shadow outlined in red is the adult in blue albeit in reverse since it is a reflection. The glare of the blinds are reflected too although in white.
To my eyes, there is no "shadow"--that's the normal room (as the man is blocking the glare-y light for that location), the part not suffering from the reflection. What we see here cannot be a reflection of the sensor off the rear element, as that reflection looks like the sensor (and is in the middle of the frame, and you wouldn't see it at f2.8), but here it is a reflection of the blinds off of one of the elements (or, in my opinion, off of the UV filter).
01-18-2011, 04:08 AM   #7
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QuoteOriginally posted by macTak Quote
or, in my opinion, off of the UV filter).
Like OP said, filter or no filter, same thing. The angle/position of the projected reflection, does make it appear to be caused by the interaction of rear element and sensor.
01-18-2011, 07:24 AM   #8
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Thanks for all the responses

Thanks to everyone for the responses. I've got some things to try now... as soon as I see a bright enough day here in the gray of Minnesota's January.

I'll post back with what I discover.

Josh

01-18-2011, 12:22 PM   #9
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QuoteOriginally posted by Ex Finn. Quote
The angle/position of the projected reflection, does make it appear to be caused by the interaction of rear element and sensor.
I think you are confused between sensor flare (reflection of the sensor itself) and simply internal reflection. Sensor flare (off the rear element) does not take an angle, period, and only shows up when stopped down more. It also looks completely different--it never looks like anything in the image, but appears as a bright spot in the center of the image. Only a couple of lenses out there have been demonstrated to cause this (a flat back element is a factor), most notably the Tamron adaptall2 90mm macro. You likely have never experienced a case of sensor reflection, which is different from internal reflection in glass elements and/or filters, which is going on here.

See here for a shot or two showing what sensor flare looks like:
https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/93388-tamron-e...ccd-flare.html

I could even post a couple examples from my time with that Tamron lens to demonstrate. As to what the OP says regarding filters, I do not know if the OP actually took test pictures with this or not (sometimes flare shows up through the viewfinder that does not occur in the actual pictures), so the OP should make sure that the UV filter really isn't the culprit, as it is the most likely cause of such an effect (or at the least will amplify it). Also, to the OP, what specific UV filter are you using?

Last edited by macTak; 01-18-2011 at 12:43 PM.
01-18-2011, 01:21 PM   #10
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I guess if "flare" is similar in definition to "reflection".
I see a perfectly reversed reflection.
01-18-2011, 08:49 PM   #11
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Thanks all for pointing me in the right direction

Well - I think I figured it out.

Color me embarrassed. Apparently I didn't actually _take_ a picture without the filter on (as much as I thought I had).

Clearly (see images below) the reflection is coming from the filter. (Just removed the filter, no hood used.) I'll be interested to try again when I get some decent light to see if I can replicate the very distinct reflections, but for now, I'll be shooting sans filter. (The filter was labeled "Rokinon UV Japan" - I understand Ricoh manufactures(d) the Rokinon line - it doesn't appear that it has any kind of coating on the glass.)

Again - thanks all for pointing me in the right direction. Glad to see that it's not reflection from the sensor itself.

Josh
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01-18-2011, 11:13 PM   #12
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QuoteOriginally posted by jdbraun Quote
Well - I think I figured it out.

Color me embarrassed. Apparently I didn't actually _take_ a picture without the filter on (as much as I thought I had).

Clearly (see images below) the reflection is coming from the filter. (Just removed the filter, no hood used.) I'll be interested to try again when I get some decent light to see if I can replicate the very distinct reflections, but for now, I'll be shooting sans filter. (The filter was labeled "Rokinon UV Japan" - I understand Ricoh manufactures(d) the Rokinon line - it doesn't appear that it has any kind of coating on the glass.)
You shouldn't feel bad, that's what I expected was the case, and I'm glad it is resolved. Yes, that is a lousy, uncoated UV filter and will create those reflections and may lower IQ in any lighting. You are best off, in my opinion, permanently ditching the UV filter--the hood I recommended provides excellent protection for the front element of the lens if that is your concern.
01-19-2011, 03:54 AM   #13
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Hey, try it without the filter on. Glad you figured it out.
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